I have had the Nexplanon implant since December 2013. I bled for a few weeks in January and then?

... none in February and again for a few weeks in March. Since then I have had no bleeding. I had unprotected sex 4 days ago, 2 days ago, and this morning. Today I went to the restroom and realized that I had a darkish Brown discharge. No blood in my underwear, I only saw it when I wiped. I did not bleed during or after sex, I even went to school after for 5 hours and just saw it when I came home. What does this mean? Could I randomly spot like this after not bleeding for 5 months? Or could this be implantation bleeding?